1. Field of the Invention
This invention relates to a method and apparatus for the enhanced recovery of petroleum. Its application could increase the recoverability of oil originally in place from the 30 to 40 percent now common, using current technology, to as much as 80 percent or more. Immediate use of this invention in newly discovered reservoirs, as well as in those now producing, could greatly accelerate the rate of oil production in the United States; also, it could bring back into production many "depleted" oil reservoirs that have been produced to abandonment using current technology.
2. Description of the Prior Art
Hydropressured aquifers are porous, permeable water-bearing formations in which the interstitial fluid pressure reflects the weight of the superincumbent water column, unconfined above, and open to the atmosphere. The depth-pressure gradient is mainly a function of the dissolved solids content of the formation water, and may range from about 0.3 to about 0.5 pound per square inch per foot of depth.
Geopressured aquifers are not open to the atmosphere, having been compartmentalized by faulting, and their fluid pressure reflect a part of, or all of, the weight of the superincumbent rock deposits. The depth-pressure gradient is mainly a function of rate of leakage, or fluid escape, from the aquifer system, and may range from about 0.5 to about 1.0 pound per square inch per foot of depth.
Geopressured aquifers exist along the Gulf Coast of the United States and in many other places throughout the world where sedimentary deposits have been rapidly buried. Due to the high pressures found in geopressured aquifers, if a well is drilled into the aquifer, water will flow to the surface of the ground in artesian fashion.
Natural gas may be present in geopressured aquifers in any of these forms:
(1) gas dissolved in the water; PA1 (2) free gas dispersed in water within the rock pores; and PA1 (3) a free gas phase present within the rock pores and separate from the water. PA1 1. Petroleum crude is highly soluble in water of low salinity (less than 50,000 mg/l) at elevated temperatures. Solubility increase is gradual to about 100.degree. C. (212.degree. F.) and rapid at higher temperatures because of a change in the solution mechanism (Price, 1976, p. 237) (see FIG. 2). PA1 2. The aqueous solubility of the least soluble compounds of petroleum increase most rapidly with rising temperature, above 100.degree. C. (212.degree. F.) (Price, 1973). PA1 3. The aqueous solubility of petroleum crude in low salinity water is greatly increased at elevated pressure and temperature by saturating the water with carbon dioxide and hydrocarbon gases (mainly methane) (Bray and Foster, 1979). PA1 4. The aqueous solubility of natural gas (methane) increases rapidly with pressure and temperature above 4,000 psi and 150.degree. C. (Sultanov et al, 1972) and exceeds 100 standard cubic feet per barrel of water (scf/bbl) at 9,000 psi and 221.degree. C.; exceeds 200 scf/bbl at 10,000 psi and 280.degree. C.; 300 scf/bbl at 27,000 psi and 280.degree. C.; 400 scf/bbl at 12,000 psi and 316.degree. C.; and 500 scf/bbl at 23,000 psi and 316.degree. C. The maximum solubility measured by Price (1979) was 828 scf/bbl at 28,610 psi and 354.degree. C. (see FIG. 3). PA1 5. Formation waters of the geopressure zone, in geologically young petroliferous basins (of Mesozoic or Cenozoic age), are universally saturated in methane, and have temperatures generally above 100.degree. C. PA1 6. Almost all (99 percent) of the oil produced in the northern Gulf of Mexico basin was recovered from reservoirs having initial temperatures less than 150.degree. C. (302.degree. F.) (Fertl and Timko, 1973). PA1 7. More than 90 percent of the oil that has been produced in the northern Gulf of Mexico basin was recovered from reservoirs having initial fluid pressures reflecting pressure gradients less than 0.7 psi/ft. PA1 8. The method and apparatus of this invention also depend heavily upon the principles of oil displacement defined by the relative permeability concept (Buckley and Leverett, 1942) (see FIG. 4).
The natural gas contained in aquifers is commonly 95-98% or more methane.
Publications which relate to the background of this invention and which are referred to herein are as follows:
1. Craft and Hawkins, "Applied Petroleum Reservoir Engineering," Prentiss-Hall, Inc., Englewood Cliffs, N.J., 1959. PA0 2. Doscher, "Tertiary Recovery of Crude Oil," in The Future Supply of Nature-Made Petroleum and Gas, Pp. 455-480: Proceedings of the First UNITAR Conference on Energy and the Future, 5-16, July, 1976, R. F. Meyer, Ed. Pergamon Press, New York, 1977. PA0 3. Jones, "The Role of Geopressure in the Fluid Hydrocarbon Regime," in Exploration and Economics of the Petroleum Industry, V. 16, Pp. 211-227, Matthew Bender & Company, New York, N.Y., 1978. PA0 4. Hocott, "Enhanced Oil Recovery: What of the Future?" in the Future Supply of Nature-Made Petroleum and Gas, Pp. 389-396: Proceedings of the First UNITAR Conference on Energy and the Future, 5-16 July, 1976, R. F. Meyer, Ed. Pergamon Press, New York, 1977. PA0 5. Caudle, "Secondary Recovery of Oil," in The Future Supply of Nature-Made Petroleum and Gas, Pp. 397-410: Proceedings of the First UNITAR Conference on Energy and the Future, 5-16 July, 1976, R. F. Meyer, Ed. Pergamon Press, New York, 1977. PA0 6. Myers, "Differential Pressures, A Trapping Mechanism in Gulf Coast Oil and Gas Fields," Gulf Coast Association of Geological Societies, V. 18, Pp. 56-80, 1968. PA0 7. Price, "Aqueous Solubility of Petroleum as Applied to its Origin and Primary Migration," American Association of Petroleum Geologists Bull., V. 60, No. 2, Pp. 213-244, 1976. PA0 8. Price, "The Solubility of Hydrocarbons and Petroleum in Water as Applied to the Primary Migration of Petroleum," Ph. D. Dissertation, Univ. of California, Riverside, 298 p., 1973. PA0 9. Bray and Foster, "Process for Primary Migration of Petroleum in Sedimentary Basins (abs.)," American Association of Petroleum Geologists Bull., V. 63, No. 4, Pp. 697-698, 1979. PA0 10. Sultanov et al, "Solubility of Methane in Water at High Temperatures and Pressures," Gazovaia promphlennost, V. 17, No. 5, Pp. 6-7, 1972. PA0 11. Price, "Aqueous Solubility of Methane at Elevated Pressures and Temperatures," American Association of Petroleum Geologists Bull., V. 63, No. 9, Pp. 1527-1533, 1979. PA0 12. Fertl and Timko, "How Downhole Temperatures, Pressures, Affect Drilling," World Oil, Feb. 1, Pp. 47-50, 1973. PA0 13. Buckley and Leverett, "Mechanism of Fluid Displacement in Sands," Petroleum Transactions, American Institute of Mining and Metallurgical Engineers, V. 146, p. 107, 1942. PA0 14. MacElvain, "Mechanics of Gaseous Ascension Through a Sedimentary Column," in Unconventional Methods in Exploration for Petroleum and Natural Gas, Pp. 15-28, Institute for the Study of Earth and Man, Southern Methodist Univ., Dallas, Tex., 1969. PA0 15. Farr, "How Seismic is Used to Monitor EOR Projects," World Oil, V. 189, No. 7, December 1979. PA0 16. Ritch and Smith, "Evidence for Low Free Gas Saturations in Water-Bearing Bright Spot Sands," Pp. 1-11: Proceedings of Seventeenth Annual Logging Symposium SPWLA, 1976.
The maximum efficient rate at which oil can be recovered from a reservoir, and the recoverable percentage of the original oil-in-place may, or may not, be dependent upon the rate at which the reservoir is produced. Recovery from true solution gas-drive reservoirs by primary depletion is essentially independent of both individual well rates and total reservoir production rates. Recovery from very permeable, uniform reservoirs under very active water drives may also be essentially independent of the rates at which they are produced (Craft and Hawkins, 1959, p. 197).
When oil is displaced immiscibly from a porous rock, such as by gas or water, a residual oil saturation is reached beyond which no more oil flows out of the individual pores. At this stage, the oil is no longer in continuous phase, having been coalesced by capillary forces into isolated, discrete droplets which cannot be displaced by the viscous forces available in the reservoir. It is this break-up of the continuous filaments of the oil phase that enhanced oil recovery processes seek to reverse or to prevent, if inaugurated soon enough (Hocott, 1977, p. 390).
Several methods for the enhanced recovery of petroleum from watered-out, pressure-depleted oil reservoirs are now in use, but none has yet proved commercial from an economic standpoint, except for certain terminal installations (Hocott, 1977, p. 394). Enhanced recovery, sometimes called tertiary recovery, may not be feasible where reservoir damage has occurred during primary or secondary oil recovery operations, or where the remaining residual oil saturation is too low. The operator who wishes to improve appreciably the ultimate recovery of oil from a producing reservoir should initiate enhanced recovery operations as soon as possible. Methods of enhanced recovery of petroleum that can begin during conventional waterflood operations are of special importance. Nearly half of the oil now produced in the United States comes from waterflood projects (Caudle, 1977, p. 397).
Enhanced oil recovery methods now in use flush the reservoir with polymers, carbon dioxide, surfactants, and solubilizers, using the following guidelines: (1) interfacial tension is increased, if possible, to enhance the effectiveness of waterflooding; (2) where oil saturation is high and connate water saturation is low, enhanced recovery by water drive is favored; and (3) where connate water saturation is high, recovery by gas drive is favored. Other methods create and drive a fireflood through the "depleted" oil reservoir using air-injection wells in which heated and mobilized residual oil moves to production wells for recovery, or they flood the "depleted" oil reservoir with steam, heating and mobilizing the oil, and driving it to recovery wells. In California, more than 10 percent of the production of oil is now by steam flood.
All of these enhanced oil recovery methods are costly and complicated. Those requiring expensive chemical additives usually fail or are marginally successful, because heterogeneity of texture and permeability in reservoir rocks makes prediction of flow path at project scale difficult or impossible.
New knowledge regarding the migration and accumulation of petroleum in deep sedimentary basins which is applied in the enhanced recovery of oil in accordance with the invention include:
Although many methods and types of apparatus for enhanced recovery of oil have been patented, none describe the method and apparatus of this invention. The method of U.S. Pat. No. 2,736,381 to J. C. Allen granted Feb. 28, 1956, and assigned to The Texas Company, involves a downhole cross-connection of a high-pressure dry gas reservoir with a lower pressured condensate reservoir, resulting in increased production from other wells completed in the condensate reservoir. No mention of water is made in that patent.
U.S. Pat. No. 3,258,069 to C. E. Hottman granted June 28, 1966, and assigned to Shell Oil Company, discloses completing a well into an overpressured water-bearing reservoir and transporting superheated water from the reservoir into the injection tubing string of an oil-bearing reservoir, evaporating some of the water in the injection tubing string, and producing oil displaced by the injection of water and steam from an adjacent production well.